Acute renal failure affects approximately 5% of all hospitalized patients, causing great morbidity and mortality, and commonly results from sepsis. Preliminary data indicate that LPS-induced ARF, a model of septic renal dysfunction, depends on the action of TNF on its receptor TNFR1 in the kidney, and is associated with renal endothelial apoptosis, inflammation, and vascular leak both in vivo and in vitro. Specific Aim 1 will use primary culture of mouse renal endothelial cells (ECs) to determine the mechanisms of TNF-induced endothelial barrier dysfunction, via use of pharmacologic inhibitors and an siRNA approach, and will study the effect on cytoskeletal disruption caused by TNF. Specific Aim 2 will determine the role of this endothelial barrier dysfunction in the course of LPS-induced acute renal failure (ARF) using transgenic mice overexpressing and deficient in the endothelial isoform of myosin light chain kinase (MLCK). Sphingosine-1-phosphate analogues, which maintain endothelial barrier integrity, will also be administered to determine the effect on LPS-induced ARF. Renal cross-transplantation will be used as a tool to define the contribution of renal as opposed to systemic endothelial injury, and chronically implanted blood pressure and renal flowprobe devices will allow real-time measurement of intrarenal hemodynamics in response to the above manipulations in the setting of endotoxemia. Specific Aim 3 will test the hypothesis that TNF-induced caspase activation amplifies renal endothelial inflammation, and will define mechanisms through which this cross-talk occurs, which may involve MLCK and other cytoskeletal events. Specific Aim 4 will use endothelial-specific knockout mice lacking caspase-8, and endothelial-specific transgenic mice with nonfunctional NF-?B, to distinguish the respective contributions of endothelial apoptosis and inflammation to the outcome of LPS-induced ARF. Together, this project will develop tools which should allow greater understanding of the importance and mechanism of endothelial injury and its role in septic ARF.